When Source Referencing is enabled in a Project, a 'Sources' tab appears at the top of the edit form when editing an Object or Category. Here one or more Types that have been set as 'Sources' in the Project settings can be selected and one or more Objects can be selected. Each reference can be supplemented by a string.
The same options are available when clicking on the grey 'note' button shown next to every Object Description, Sub-Object, and Sub-Object Description. Go to the tab 'Source' in the popup that opens.